hacked, strange link on my blog

i have discovered today that at the top of my blog is a few lines of text and a link, obviously some sort of hack and its a gambling site i think. i cannot see this link when i am in the dashboard, but can only see it when i view the blog as a reader would. if i delete a post it just appears on the next post. i have no idea how to get rid of it, i’m not great at these things. i have changed my password. any advice welcomed. thanks